Despite plasticity, heatwaves are costly for a coral reef fish

Climate change is intensifying extreme weather events, including marine heatwaves, which are prolonged periods of anomalously high sea surface temperature that pose a novel threat to aquatic animals. Tropical animals may be especially vulnerable to marine heatwaves because they are adapted to a narrow temperature range. If these animals cannot acclimate to marine heat waves, the extreme heat could impair their behavior and fitness. Here, we investigated how marine heatwave conditions affected the performance and thermal tolerance of a tropical predatory fish, arceye hawkfish (Paracirrhites arcatus), across two seasons in Mo'orea, French Polynesia. We found that the fish’s daily activities, including recovery from burst swimming and digestion, were more energetically costly in fish exposed to marine heatwave conditions across both seasons, while their aerobic capacity remained the same. Given their constrained energy budget, these rising costs associated with warming may impact how hawkfish prioritize activities. Additionally, hawkfish that were exposed to hotter temperatures exhibited cardiac plasticity by increasing their maximum heart rate but were still operating within a few degrees of their thermal limits. With more frequent and intense heatwaves, hawkfish, and other tropical fishes must rapidly acclimate, or they may suffer physiological consequences that alter their role in the ecosystem.

Methods
Hawkfish (Paracirrhites arcatus) were collected in Mo'orea, French Polynesia during the Austral winter (2019) and Austral summer (2022). They were acclimated to one of five treatments (27, 28, 29, 31, or 33°C) for one week and tested for metabolic performance metrics, including maximum metabolic rate (MMR), standard metabolic rate (SMR), and costs of digestion (specific dynamic action, SDA) using custom-made respirometry chambers. A subset of fish was also tested for thermal tolerance using an acute cardiac thermal tolerance test. In addition, wild fish were also caught and tested for acute cardiac thermal tolerance. Metabolic data were processed in R and heart rate data were processed in LabChart. Please see the published manuscript for detailed methods.